Still I couldn't help thinking of New Zealand Flax (Phormium tenax, Phormim cookianum) as a substitute when hearing an expert explain how resistant to breakdown plastic is in the environment. There was once a commercial Flax fiber industry in New Zealand, mainly for rope production, but the product was considered inferior to sisal or manila because it swelled when wet and wore badly(1), still it will survive considerable exposure to water as evidenced by the fact that Maori made fishing nets 1000 meters long out of it. I used to tie stakes for plants with strips of flax torn straight from the plant, they lasted many months outside.
Flax can be grown in a wide range of conditions that might not be good for much else , Phormium cookianum grows on vertical coastal cliffs.
There are probably a number of other "second-rate" fiber crops that would also decompose quickly.
1. Encylopaedia of New Zealand Vol 1. 1966. pp 704-705